Pseudogenes and the "De-evolution" of Human Olfactory Receptors
Overview of Pseudogenes: Genomic evolution can occur via mutations, exon shuffling, gene duplication and a variety of other means. One particular mechanism for a genome-wide "shift" is the replacement of functional genes with pseudogenes. Pseudogenes are essentially defunct genes that are derived from their functional counterparts, and can crop up within a genome when an insertion or deletion of one or multiple nucleotides occurs, resulting in the gene no longer being able to code for the expected protein. A premature stop codon or a mutation in the promoter region can also cause the proliferation of pseudogenes. The Human Sense of Smell: Unlike most mammals, human beings lack a refined sense of smell. Though we are able to perceive the aroma of food or the repugnant stench of garbage (this occurs when external scents bind to the cilia of our noses, triggering neurons that send an "odor identification" message to the brain), only "immediate" smells within close proximity to our nose can be detected, unlike dogs or raccoons--both examples of mammals with a keen sense of smell that allows them to detect predators or prey from hundreds of feet away. Humans' poor sense of smell was not always the norm. In fact, we are equipped with the same number (1,000) of olfactory receptor (OR) genes as other primates. The big difference lies in the fact that 54% of human olfactory receptors genes are non-functional pseudogenes. In most other mammals, the percentage of non-functional pseudogenes is just 25-32%. Thus, a significant discrepancy exists here between what humans are biologically capable of smelling, and what we are actually able to smell. ' Studying the Proliferation of Pseudogenes in Human ORs:' One study that focuses on the olfactory receptor genes in humans is that conducted by GM Hughes et al (University of Illinois), which compares the size of the olfactory bulb in the fossilized skulls of Neanderthals (early human ancestors existing 30,000 years ago) versus bulb size in contemporary humans. The bulb is located in the olfactory epithelium of the nasal passage, and it was determined to be double the size 30,000 years ago than in modern humans. Thus, the study authors concluded that an evolutionary change in the relative size and importance of the olfactory region is likely. To further their hypothesis, the authors compared 853 modern human olfactory receptors to 813 chipanzeee olfactory receptors (using HORDE--The Human Olfactory Receptor Data Exploritorium database). Chimps were used as comparison, since they are a close relative to both Neanderthals and modern humans. They found that of the 853 human ORs, 466 were pseudogenes, whereas for the chimps, only 233 of the 813 were pseudogenes. As stated in the study: "approximately 60% of human OR genes carry one or more coding region disruptions and are therefore considered pseudogenes...In chimpanzees, the fraction of OR pseudogenes is only approximately 30%. However, both humans and other apes have a significantly higher fraction of OR pseudogenes than do the mouse or the dog (approximately 20%). Thus, there has been a decrease in the size of the intact OR repertoire in apes relative to other mammals, with a further deterioration in humans." As is apparent, both humans and their closest-living relative, chimpanzees, have experienced an evolutionary decrease in the number of functional olfactory receptor genes, and an increase in the number of pseudogenes. The Impetus behind Humans' Waning Sense of Smell: While a definitive answer has not yet be identified, scientists have reasoned that human beings have lost their sense of smell as a result of developing other, more useful sensory receptors throughout the body. For one, humans have a larger, more complex brain, and this has allowed us to apply "reasoning schemes" to situations in which scent may have been important in the past. For instance, rather than being able to "smell" where food or water might be, we are able to "reason" and strategize to obtain these necessities. Secondly, we have developed a greater relience on visual and auditory cues than on smell, and so seeing and hearing have become more useful to our survival than smell. Thus, it is no longer necessary for our noses to be so highly-attuned to the environmental, causing the human genome to evolve in such a way that has made a keen sense of smell unnecessary. References: Beale, Bob. "Why Humans Lost their Sense of Smell." ABC Science, 2003. Gilad Y, Wiebe V, Przeworski M, Lancet D, Pääbo S. "Loss of Olfactory Receptor Genes Coincides with the Acquisition of Full Trichromatic Vision in Primates." 2003. PLoS Biol, edition 5 Hughes GM, Teeling EC, Higgins DG. "Loss of Olfactory Receptor Function in Hominin Evolution." PLoS ONE 2013. Edition 84714. Yashihitu, Niihimura. "Evolution of Olfactory Receptor Genes in the Human Genome." Proceedings of the National Academy of Science, 2011.